I do not understand this code in python (help)

Question

So the code is

def mystery(n):
    a, b = 0, 1
    while (a < n):
        print(a)
        a, b = b, a + b

The bit I do not understand very well is how a, b = b, a + b works. It seems really difficult to understand for me. I struggle to comprehend how variables work with listing involved. Could someone shed some light on what sequence is being produced and how the numbers tally up.

The sequence is meant to go

0

1

1

2

3

5

8

13

21

34

etc

I would be really greatful, thanks in advance!

Note that if you search for `def mystery(n) site:stackoverflow.com` you will find nearly 25,000 almost identical questions... — jonrsharpe, Mar 17 '15 at 11:58
@jonrsharpe OMG, and a lot of them are about this exact algorithm. — Yatharth Agarwal, Mar 17 '15 at 12:01
@YatharthROCK yes, it's a pretty common introductory programming exercise — jonrsharpe, Mar 17 '15 at 12:01

score 2 · Answer 1 · answered Mar 17 '15 at 11:56

2

In a, b = b, a + b, the expressions on the right hand side are evaluated before being assigned to the left hand side. So it is equivalent to:

c = a + b
a = b
b = c

Which is actually swapping in your case so , OP:

a, b = 0,1

will evaluate to b=0 and a=1

answered Mar 17 '15 at 11:56

Tushar Gupta

15,504
1
29
47

I do not understand this code in python (help)

1 Answers1